LU-10070 lod: SEL: split lod_del_layout

LU-10070 lod: SEL: split lod_del_layout

SEL deletes layout components as part of other operations,
rather than only as a separate delete operation.

So we split lod_layout_del in to a function that prepares
the layout and one that writes it out.  The prep_layout
function will be used in later patches.

Cray-bug-id:  LUS-2528
Signed-off-by: Patrick Farrell <paf@cray.com>
Change-Id: I5d7270db9a8d9bc94f4571906ed9e2d4a17a151b
Reviewed-on: https://review.whamcloud.com/33780
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>